The invention relates to display and recording devices, particularly those in which images or patterns are formed by means of the electrophoretic migration of particles.
The use of the electrophoresis phenomenon for use in display devices containing an collidal suspension layer between electrodes is known in the prior art, such as represented by U.S. Pat. Nos. 3,612,758, 3,668, 106 and 4,071,430. These patents are hereby incorporated by reference to provide the basic information on the chemistry and technology of electrophoretic displays.
A simple electrophoretic image display (EPID) device is known, which is a passive, scattering-type display. This EPID device is implemented as a cell which consists of a suspension of pigment particles in an organic liquid held between two electrodes, at least one of which is transparent. In one embodiment, the pigment particles are negatively charged with respect to the liquid. If a positive potential is applied to an electrode in the suspension, the pigment particles will be attracted to that electrode. A negative potential on an electrode repels the particles.
One side panel of the device is made of a transparent material such as glass, coated with a transparent, electrically-conducting layer. When light-colored pigment is attracted to this side panel electrode, the observer looking at the panel sees the reflected color of the pigment. Due to the dark-colored dye in the suspending medium, the pigment at the back of the cell is obscured by the opaque dye and the observer sees only the reflected color of the dye. When the polarity of the voltage on the electrodes is reversed, the position of the colors is reversed. The described EPID display cell also possesses "memory" since the particles remain on the electrodes after the applied voltages are removed due to chemical, electrical, or van der Waals forces.
Such EPID devices described above are not practical for certain application since they lacked a fixed, specific switching threshold for the transport of pigment from one side of the cell to the other.
U.S. Pat. No. 3,612,758 suggests the use of an additional conductive layer for enhancing the threshold migration of pigment particles in the electrophoretic suspension. Although the patent alleges an effect of threshold migration of pigment particles, the disclosure admits that the exact mechanism for such an effect is not fully understood. If such an effect does exist (and there is no evidence from the prior art that it does), it would not be a natural threshold, nor would it be easily reproducible or lend itself to a commercial implementation for electrophoretic display devices. In addition, the lack of a sharp threshold characteristic will lead to slow addressing times and serious half-select problems. A half-select condition exists for those display elements that are not addressed but are on portions of a selected row or column electrode. These elements experience approximately half the applied voltage and in the case of this prior art, pigment in these elements would be partially transported across the cell, causing noticeable changes in the displayed information. Such disadvantages make the prior art suggestions for an X-Y addressing technique of electrophoretic display devices impractical.
In a paper entitled "Recent Progress In Electrophoretic Displays", by Andrew L. Dalisa and Roger A. Delano, and published in the "Digest of Technical Papers of the International Symposium of the Society for Information Display" held in San Diego, Calif. in May, 1974 a rudimentary control grid EPID cell was reported. The control grid, defined in the published report, was a "wire grid" positioned between and electrically isolated from the two electrode surfaces reported to be normally biased positive with a voltage with approximately 1/2 volt with respect to electrode A. The rudimentary configuration shown in the published paper (particularly FIG. 5) does not describe X-Y addressing, and moreover the structure of the relatively simple schematic diagrams shown in the paper does create an actual threshold but does not result in an easily reproducible or commercially implementatable electrophoretic display device. In short, the prior art configurations were impractical to implement X-Y addressing in electrophoretic display devices.